Spot currency. What do you mean by huge notional sizes? IB is paying 4% on RUB on RUB cash > 700000 RUB = 11800 USD. I am not a forex trader at all, so I actually don't know if IB is screwing us. It is just a way to hedge my USD exposure not a trade per se.
You are getting 4% when actual rates is closer to benchmark of 9% for huge risk in RUB. Think about it this way, you are investing in a risky instrument and straightaway losing 5% per annum just by holding it.